Contactors are devices which can be used for switching power circuits. They can generally handle loads drawing high currents. As such, they may for example be used to control or switch electric motors, electric drives and the like.
Contactors are different to circuit breakers in that they are not intended to interrupt short circuit currents, as the contact regions may otherwise suffer severe damage due to arcing. Arcing can also occur when switching at nominal currents. To reduce arcing, the contacts of the contactor can be placed under vacuum or in an inert gas.
To avoid the detrimental effects of arcing when short circuit currents occur, the contactor can be maintained in a closed position. This is particularly true for subsea applications. In such application, the contactor may for example be placed in a pressurized enclosure, such as a subsea canister, which is filled with dielectric liquid that is balanced to the outside pressure. Arcing can lead to a combustion and extensive contamination of the dielectric liquid. Accordingly, it is desirable to keep the contactor closed to prevent arcing.
Conventional contactors are generally not suitable for such operation. If the contactor remains closed, there will be excessively high currents in the contacts. These currents will result in a blow-off force or Slade force which acts on the contacts in a direction that separates the closed contacts and reduces the contact force/pressure. This reduction leads to a degradation of the electrical contact and to an increase in resistance. Due to the high short circuit current through the contact, the increased resistance leads to excessive heating and as a consequence to the welding of the contacts. The contacts can thus not be opened any more, rendering the contactor inoperable. This is particularly problematic for subsea applications, since removal of the contactor from the sea floor and its replacement are technically challenging and very cost intensive tasks.
It is thus desirable to provide a contactor which can remain in a closed state even in the presence of a short circuit current without suffering severe damage and which is still operable after clearance of the fault.